Deep-water tendon and riser systems are often subjected to severe fatigue loading from waves, currents and vessel movements. The girth welds between successive lengths of pipe or at pipe terminations represent fatigue-critical features where failure would be catastrophic. Hence, validation fatigue testing by full scale pipes of the most critical welds are often performed to ensure adequate quality and/or to document a better S-N curves than those available in standards today like DNVGL-RP-C203 [1] and BS7608 [2]. To better understand the fatigue performance with respect to identify trends, dependencies and critical features that influence the fatigue performance, a JIP on Fatigue of Girth Welds were initiated in 2011. Two phases have been conducted and a total of 1700 full scale one sided girth welds, mostly run by Stress Engineering, have been statistically analyzed. The test data has been interrogated to investigate the effect of as-welded condition, OD ground, OD/ID ground, un-reeled pipe, reeled pipe, thickness and effect of misalignment. Based on these analyses, new S-N curves for risers and pipelines have been included in DNVGL-RP-C203 for non-reeled girth welds. This paper presents the findings and trends from the JIP work which has been the rationale for the updates of girth welds in section 2.10 in DNVGL-RP-C203 2016 edition.

This paper is concerned with the challenges related to steel design under Arctic conditions where both loading and temperature have been discussed in relation to material requirements. Today there is a lack of rules and standards for selecting steel materials for bulk engineering for a lower design temperature than −10°C (NORSOK N-004 [1] allows down to −14°C). Both ISO 19902 Steel Structures [2] and NORSOK N-004 Design of steel structures make reference to EN10225 “Weldable structural steels for fixed offshore structures technical delivery conditions [5]” where steel materials are Charpy tested at a lowest test temperature of −40°C and proven for a design of −10°C. Hence, one major challenge for designers are to specify adequate toughness requirements at an early stage of the design process for low temperature applications. Both NORSOK N-004[1] and ISO 19902[2] provide requirements to load combinations that need to be fulfilled, however the relationship between various load types and temperature is not mentioned in any of these standards. Thus, in the design stage the material needs to demonstrate adequate toughness where loading and temperature are treated independently. For the offshore industry, the main question is the balance between materials requirements and cost-effective solutions, and how to address this within an overall design perspective in order to avoid brittle failure. This paper discusses some of these challenges with the aim of starting a focused process leading up to a clear interpretation of the implications of overall design philosophies, necessary in order to define consistent materials requirements ensuring that brittle fracture is not going to represent a significant threat to the structural integrity. The material recommendations provided in the paper are based on the latest research results from the Arctic Materials project (2008–2017) managed by SINTEF and supported by the industry.

A large diameter high strength titanium free-hanging catenary riser was evaluated by the Demo 2000 Ti-Rise project, from initiative of the Kristin Field development license. In order to reduce the uncertainties related to the schedule, cost, and special technical issues identified in the work related to a similar riser for future installation on the A˚sgard B semi-submersible platform, a fabrication qualification of a full scale riser in titanium was run. Several full-scale production girth welds were made in an in-situ fabrication environment. The welding was performed on extruded titanium grade 23 (ASTM) pipes with an ID of 25.5″) and wall thickness of 30 mm. The main challenge was to develop a highly productive TIG orbital welding procedure, which produced welds with as low pore content as possible. It is well known that sub-surface pores often are initiation sits for fatigue cracks in high strength titanium welds. This paper describes how a greatly improved productivity was obtained in combination with a high weld quality. NDT procedures were developed whit the main on the reliability to detect and locate possible sub-surface weld defects, volumetric defects such as pores and tungsten particles and planar defects such as lack of fusion. The results from the actual Non Destructive Testing (NDT), the mechanical testing, and the fatigue testing of the subjected welds are presented. The response of the catenary is optimised by varied distribution of weight coating along the riser’s length. A satisfactory weight coating with sufficient strength, bond strength, and wear properties was developed and qualified. The riser is planned to be fabricated from extruded titanium pipes, welded together onshore to one continuous piece. The field coating is added and the riser is loaded into the sea and towed offshore and installed.

The paper describes the main topics and findings of the JIP Management of Riser Contact, performed as a co-operation between Aker Kvaerner Technology and Det Norske Veritas, and sponsored by 5 major oil companies. The overall objective is to establish a design practise that properly accounts for contact between top-tensioned risers in operational conditions typical for the major deepwater locations.The results, findings and recommendations for design and analysis are embedded in a Design Guideline document, reflecting industry consensus on methodology for engineering analysis and design for riser collision. The Design Guideline document constitutes the first steps towards a formal DNV Recommended Practise document, and includes a state-of-theart introduction to the main building stones for riser collision assessment.So far, design codes have not allowed riser collision under normal operational or even extreme conditions. In the Design Guideline document, riser collisions are allowed provided that the riser array is subjected to state-of-the art analyses and sufficient fatigue (FLS) and ultimate (ULS) capacity are documented.The paper discusses the main items regarding riser interference comprising hydrodynamic interaction, local response of risers in contact and acceptance criteria for design.

Deep-water tendon and riser systems are often subjected to severe fatigue loading from waves, currents and vessel movements. The girth welds between successive lengths of pipe or at pipe terminations are the locations most vulnerable to fatigue damage and accurate and reliable assessment of the fatigue performance of these welds is of significant importance. These welds are normally designed on the basis of appropriate S-N curves. In addition, it is common practice to perform an Engineering Critical Assessment (ECA) based on fracture mechanics fatigue crack growth principles, principally in order to establish acceptance limits for weld flaws that are consistent with the fracture toughness of the welds and the required fatigue life. However, for the most important case of fatigue failure from the weld toe, the mode of failure principally covered by the S-N curves, a conventional ECA invariably results in tolerable weld toe flaws that are too small for reliable detection by currently available NDE methods. In fact, this situation is not unreasonable since it is known from careful metallurgical examinations that weld toe fatigue cracks initiate at tiny (<0,5 mm deep) sharp imperfections that are an inevitable consequence of welding, whereas the detection limit for even the most sensitive NDE techniques is around 1–2 mm. Hence this paper discusses different models for including crack initiation and early growth of defects in girths weld, in addition some trends found in full-scale data from an on-going girth weld fatigue JIP is shown.